<input type=button style="width:25px; font-size: 18px" value="A"></input>
<input type=button style="width:25px; font-size: 18px; padding: 0px;" value="A"></input>
On Mac OS X, two buttons above are rendered identically. In both cases, 'A' is centered in the button.
On Windows (Safari 4 and Chrome), 'A' is not centered but shifted to the right in the first button as if left-padding is applied first without considering whether there's enough room for 'right-padding.
The UA stylesheet for input button has 'padding-left' and 'padding-right' set to 6px.